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Vacuum properties of palladium thin film coatings
ARTICLE IN PRESS
Vacuum 73 (2004) 139–144
Vacuum properties of palladium thin film coatings C. Benvenuti*, P. Chiggiato, F. Cicoira, Y. L’Aminot, V. Ruzinov EST Division, CERN European Organization for Nuclear Research, Geneva 23 CH-1211, Switzerland
Abstract A recent development, carried out at CERN for particle accelerator applications, showed that a vacuum chamber coated with a thin getter film and then exposed to ambient air may be transformed into a pump by ‘‘in situ’’ heating at temperatures as low as 180 C. Heating activates the diffusion into the film of the oxygen present in the surface passivation layer. Repeated air exposure–activation cycles progressively enrich the film with oxygen, reducing its performance and shortening its operating life. To overcome this inconvenience, noble metal coatings were considered. At distinction with getters, noble metals may release all the pumped gases by heating, resulting in a practically unlimited life. Thin film coatings of palladium were studied by surface analysis, electron stimulated desorption and pumping speed measurements. These coatings were found to pump H2 and CO, even without activation by heating, but not N2 or CO2. Thin Pd and Pd–Ag films were also used as overlayers for protecting a getter film from oxidation while not impairing its H2 pumping. The result of these studies are presented and discussed. r 2003 Elsevier Ltd. All rights reserved. PACS: N 07.30.kf Keywords: UHV; Getter coatings; Pd Films; Accelerator technology; H2 absorption
1. Introduction Thin film coatings of non-evaporable getter (NEG) materials, produced by sputtering, were developed at CERN and extensively studied during the past few years [1–13]. These coatings strongly inhibit the outgassing of the underlying surface and, after activation, transform the coated surface into a pump. Activation temperatures as low as 180 C were achieved for TiZrV films, so *Corresponding author. Tel.: +41-22-7673718; fax: +41-227679150. E-mail address: [email protected] (C. Benvenuti).
rendering activation compatible with the baking temperature of copper and aluminium alloy vacuum chambers. Depending on coating characteristics, pumping speeds up to 1 l s 1 cm 2 for H2 and 10 l s 1 cm 2 for CO were measured at room temperature, together with CO surface capacities up to 1016 molecules cm 2 [12]. Ultimate pressure limitations were not detected down to 10 13 Torr, except those resulting from the presence of gases not pumped by NEG (i.e. methane and rare gases) and/or due to the measuring gauge outgassing [6]. This technology was developed for the demanding vacuum needs of particle accelerators, for which it provides a high, evenly distributed
0042-207X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.022
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pumping speed, with the additional unexpected bonus of very low secondary electron yields [13]. However, NEG coatings could be conveniently used for other UHV applications, as for instance to improve the performance of turbomolecular and sputter-ion pumps, or for standard UHV components. Furthermore, they could also be adopted for electronic devices and more generally for any vacuum sealed device such as cryogenic dewars/transfer lines or evacuated solar collectors. All the materials tested are based on the elements of the IV-B column of the periodic table, i.e. Ti, Zr and Hf. These elements provide oxygen solubility limits higher than 20% (atomic), i.e. higher by more than an order of magnitude than those of any other element at room temperature. Due to high binding energy, oxygen is not released by getters while heating. Therefore, repeated activation/atmospheric exposure cycles result in a progressive oxygen enrichment of the coating, which in turn limits its operating life. When assuming that the surface passivation layer consequent to atmospheric exposure contains the equivalent of 10 monolayers of oxygen, which is dissolved inside the coating during activation, an oxygen content at the percent level is established after five activation cycles of a film 1 mm thick. Although also grain boundaries may participate to the oxygen uptake, the importance of a high solubility limit for oxygen is evident. Due to the practical importance of these considerations, the performance variation (ageing) of NEG films, resulting from repeated activation/ air venting cycles was extensively investigated. It was shown that a 5 mm thick TiZrV film may undergo more than 50 such cycles with minor consequences [10], provided that the activation temperature is progressively increased from 200 C to 350 C. Although reassuring, this result does not remove the problem. To do so, materials with little or no reactivity for O2, N2, H2O would be required. In this case not only the coating life would be unlimited, but also heating would not be mandatory for activation. The obvious price to be paid, however, would be a highly selective pumping. Materials with these properties actually exist inside the family of noble metals. In particular,
palladium presents a negligible enthalpy of reaction with CO2 and N2, while its low value for CO and H2 allows these gases to be desorbed by heating. Palladium is particularly interesting also for its extremely high diffusivity and absorption capacity for H2, and therefore it has been privileged in our study. Platinum has also been submitted to exploratory tests, and the available results will be reported for comparison.
2. Experimental techniques Coating was carried out by magnetron sputtering making use of a central cathode consisting of single wires 1 mm in diameter. Typical coating parameters were cathode voltage 500 V, discharge gas (argon) pressure 2 10 2 Torr, applied magnetic field 200 G, substrate temperature 100 C, ( s 1 and film thickness about coating rate 0.1 A 0.5 mm. A more detailed description of the coating process may be found in Ref. [1]. Samples of different size and geometry (small samples for surface analysis and cylindrical tubes for vacuum performance evaluation) were analyzed by Auger spectroscopy, electron microscopy, pumping speed measurements and electron stimulated desorption (ESD) of neutrals. As for the production procedure, the evaluation techniques were the same already used for NEG coatings and described in more details in Ref. [1].
3. Results 3.1. Surface analysis As expected for noble metals, the Auger spectra of the air-exposed Pd films are widely different from those of most metals. The O-KLL line at around 515 eV, produced by the surface oxide, is in this case absent, even for as-received, not heated samples, showing that a stable surface passivation layer is not formed on Pd upon atmospheric exposure. However, a carbon surface contamination may be detected, very similar to that usually noticed for metal surfaces of different nature.
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3.2. ESD Important differences are also visible in the ESD results, shown in Fig. 1. In this figure the total pressure increase and the effective desorption yield variations as a function of the heating cycles for Pd, TiZr and stainless steel are compared. In all cases, the temperature dependence is much less pronounced for Pd than for the TiZr film, without any indication of activation onset. The absolute values for H2 are extremely low, while for CH4 they are similar to those of activated TiZr. The CO2 yields are much higher, also due to the lack of pumping for this gas, while the yields of CO are not measurable because in the shadow of the CO2 cracking pattern. Very
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similar results (within a factor of 2) were obtained for Pt. The origin of the dominant desorption yield for CO2 deserves a complementary comment. Since this gas is not chemisorbed by Pd, its release cannot be imputed to a direct desorption mechanism, but rather to an electron-induced cracking of the carbonaceous surface contaminants. If the sample is not heated (see Fig. 2) the desorption yields are initially about one order of magnitude higher than those measured after 120 C heating, and they decrease with increasing pumping time, reaching values 2–5 times higher after 50 h of pumping. Also in this case very similar results were obtained from Pt coatings.
Fig. 1. Dependence of Pd coating ESD results on heating temperature (heating duration 2 h), compared to that from stainless steel and TiZr coating. The samples are bare and coated stainless steel cylinders 50 cm long and 10 cm in diameter. The energy of the bombarding electrons is 500 eV, the electron current 1 mA. The reported data are apparatus-dependent whenever sample surface pumping takes place because the competing action of desorption and sample pumping are not disentangled.
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160 mm diameter, connected at the other extremity to an UHV vacuum system via an orifice of 25 l s 1 of conductance for H2. A detailed description of this system is given in Ref. [6]. The pressure measured after 24 h baking at 250 C was 2 10 13 Torr. The corresponding sticking factor for H2 was higher than 0.15. This value was obtained by Monte Carlo simulation from the pressure attenuation across the chamber measured when the sample gas is injected at one of its extremities. Since only total pressure is measured, the presence of impurities cannot be excluded. If the impurities are not pumped by the Pd film, they adversely affect the pressure gradient and decrease the calculated sticking probability. This effect is more important for long, narrow chambers and injected gases pumped with high speed. Since both these condition are present in our case, only a lower limit of the sticking probability could be given. For comparison, H2 sticking probabilities of 0.5 are reported [15]. For the same reason, the sticking probability for CO could not be obtained; according to literature, it is very close to 1 [14]. Pumping speeds for H2 were also measured for the unbaked Pd-coated chamber after different pumping times. Fig. 3 shows the H2 pumping speed versus water vapour pressure, which steadily decreases while pumping. This result indicates that heating of Pd thin films is not required for H2 pumping.
4. Pd coatings on NEG films
Fig. 2. Results obtained as described for Fig. 1 after different pumping times without sample heating. The horizontal lines correspond to the values measured after heating at 120 C.
3.3. Ultimate pressure and pumping speeds The ultimate pressure was measured at the extremity of a 2 m long Pd-coated chamber of
Although large H2 quantities may be accommodated in Pd at high pressures, due to its low binding energies both for surface adsorption (about 1 eV) [15] and for solid solution (about 0.1 eV) [16], even small H2 quantities result in dissociation pressures unacceptably high if compared to UHV standards. A better situation might be achieved if the Pd film is coated over a predeposited NEG film as already reported for Pd coatings on bulk Nb and Ta [17]. In this case H2 could migrate through the Pd to the NEG film, where it may be stored with a negligibly low dissociation pressure. For this
ARTICLE IN PRESS C. Benvenuti et al. / Vacuum 73 (2004) 139–144
Fig. 3. Hydrogen pumping speed dependence on the water vapour pressure for the Pd coated sample of Fig. 1. The measurements were carried out using a Fisher–Mommsen pumping speed dome after different pumping times in an unbaked vacuum system. Care has been taken to minimize the injected amount of H2 for pumping speed evaluation, so as not to affect appreciably the H2 surface coverage which in turn would result in a pumping speed reduction for the following measurements.
application, Pd might be advantageously replaced by PdAg alloys, which provide an easier transmission of H2 to the underlying NEG [18]. In order to explore this possibility, stainless steel vacuum chambers coated with Pd, Pd/NEG, and Pd70-Ag30/NEG, were exposed to various H2 loads (after bakeout at 120 C for 24 h) and then heated under static vacuum with a ramping rate of 60 C/h. The results are shown in Fig. 4. The Pd film coated on stainless steel, even without any H2 injection, displays a steady pressure increase for increasing temperature. At distinction with this behaviour, in the case of Pd coated on a TiZr film, heating produces initially an increase of the H2 pressure, which then decreases down to the ultimate pressure of the system. This trend is common to the H2 loaded and unloaded Pd/NEG coated chambers, although with a slightly different H2 pressure peak temperature, showing that H2 may be fully transferred at temperatures lower than 100 C. On the other hand, no pressure increase is detected for PdAg/NEG, in spite of the much larger injected H2 quantity, showing that no heating is needed in this case for transferring
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Fig. 4. Temperature dependence of the H2 pressure measured for different samples (same geometry as those of Fig. 1) submitted to different H2 loads and heated under static vacuum at a rate of 60 C/h.
H2 to the NEG film due to the low activation barrier for H2 migration from the surface into the PdAg bulk and from the latter into the TiZr film.
5. Conclusions On palladium thin film coatings exposed to ambient atmosphere a stable passivation layer is not formed. The low binding energies for H2 and CO make their pumping thermally reversible, so allowing a practically unlimited operating life to be achieved. A similar behaviour is also displayed by Pt films, which however have not been studied at the same extent. In order to compensate for the low H2 pumping capacity in the UHV pressure range Pd and PdAg coatings have been used as overlayers on NEG films. In this case both the higher sticking factor for H2 of Pd and the large H2 absorption capacity of NEGs are jointly available in a hybrid coating, which is ageing free and does not require heating for activation. Such features make Pd/NEG and PdAg/NEG particularly suited for clean UHV systems which
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are often vented to air and/or for pumping large quantities of H2 in unbaked vacuum systems, where water vapour would otherwise completely saturate the NEG film. In this case, however, the coating should be kept warm enough during operation to prevent water vapour physisorption (about 100 C). For particle accelerator applications, the hybrid coatings provide lower electron-induced desorption yields than the usual construction materials, and even than NEG coatings in the case of H2. On the other hand, no pumping is provided for CO2, and the secondary electron yields are intrinsically high (peak value of 1.4 even after heating at 350 C [19]). Before an application to a real machine could be envisaged, more work would be needed, particularly to explore the dynamic vacuum behaviour under electron/synchrotron radiation surface bombardment and the possible benefits of specific treatments to decrease CO2 desorption.
References [1] Benvenuti C, Chiggiato P, Cicoira F, L’Aminot Y. J Vac Sci Technol A 1998;16:148. [2] Benvenuti C, Chiggiato P, Cicoira F, Ruzinov V. Vacuum 1998;50:57. [3] Benvenuti C, Proceedings of EPAC 1998. URL: http:// accelconf.web.cern.ch/AccelConf/e98/PAPERS/ THZ02A.PDF.
[4] Benvenuti C, Cazeneuve JM, Chiggiato P, Cicoira F, Escudeiro Santana A, Johanek V, Ruzinov V, Fraxedas J. Vacuum 1999;53:219. [5] Benvenuti C, Chiggiato P, Costa Pinto P, Escudeiro Santana A, Hedley T, Mongelluzzo A, Ruzinov V, Wevers I. Vacuum 2001;60:57. [6] Benvenuti C, Escudeiro Santana A, Ruzinov V. Vacuum 2001;60:279. [7] Prodromides A, Scheuerline C, Taborelli M. Vacuum 2001;60:35. [8] Chiggiato P, Kersevan R. Vacuum 2001;60:67. [9] Benvenuti C, Chiggiato P, Mongelluzzo A, Prodromides A, Ruzinov V, Scheuerlein C, Taborelli M, L!evy F. J Vac Sci Technol A 2001;19:2925. [10] Benvenuti C, Proceedings of PAC 2001. URL: http:// accelconf.web.cern.ch/AccelConf/p01/PAPERS/ ROPB002.PDF. [11] Collins IR, Ruzinov V, Malyshev OB, Anashin V, Dostovalov R, Fedorov N, Krashov A, Proceedings of EPAC 2002. URL: http://accelconf.web.cern.ch/AccelConf/e02/PAPERS/WEPDO013.pdf. [12] Benvenuti C, Chiggiato P, Costa Pinto P, Prodromides A, Ruzinov V. Vacuum 2003;71:307. [13] Henrist B, Hilleret N, Scheuerlein C, Taborelli M. Appl Surf Sci 2001;172:95. . [14] Conrad H, Ertl G, Koppers J. Surf Sci 1978;76:323. [15] Behm RJ, Christmann K, Ertl G. Surf Sci 1980;99:320. . G. Physics [16] Jehn H, Speck H, Hehn W, Fromm E, Horz data, gases and carbon in metals, Pt XIX. Karlsruhe: Fachinformationszentrum Energie Physics Mathematik GmbH, 1981. p. 7. [17] Pick MA, Davenport JW, Strongin M, Dienes GJ. Phys Rev Lett 1979;43:286. [18] Noordermeer A, Kok GA, Nieuwenhuys BE. Surf Sci 1986;165:375. [19] Hilleret N. (CERN). Private communication.
Vacuum 73 (2004) 139–144
Vacuum properties of palladium thin film coatings C. Benvenuti*, P. Chiggiato, F. Cicoira, Y. L’Aminot, V. Ruzinov EST Division, CERN European Organization for Nuclear Research, Geneva 23 CH-1211, Switzerland
Abstract A recent development, carried out at CERN for particle accelerator applications, showed that a vacuum chamber coated with a thin getter film and then exposed to ambient air may be transformed into a pump by ‘‘in situ’’ heating at temperatures as low as 180 C. Heating activates the diffusion into the film of the oxygen present in the surface passivation layer. Repeated air exposure–activation cycles progressively enrich the film with oxygen, reducing its performance and shortening its operating life. To overcome this inconvenience, noble metal coatings were considered. At distinction with getters, noble metals may release all the pumped gases by heating, resulting in a practically unlimited life. Thin film coatings of palladium were studied by surface analysis, electron stimulated desorption and pumping speed measurements. These coatings were found to pump H2 and CO, even without activation by heating, but not N2 or CO2. Thin Pd and Pd–Ag films were also used as overlayers for protecting a getter film from oxidation while not impairing its H2 pumping. The result of these studies are presented and discussed. r 2003 Elsevier Ltd. All rights reserved. PACS: N 07.30.kf Keywords: UHV; Getter coatings; Pd Films; Accelerator technology; H2 absorption
1. Introduction Thin film coatings of non-evaporable getter (NEG) materials, produced by sputtering, were developed at CERN and extensively studied during the past few years [1–13]. These coatings strongly inhibit the outgassing of the underlying surface and, after activation, transform the coated surface into a pump. Activation temperatures as low as 180 C were achieved for TiZrV films, so *Corresponding author. Tel.: +41-22-7673718; fax: +41-227679150. E-mail address: [email protected] (C. Benvenuti).
rendering activation compatible with the baking temperature of copper and aluminium alloy vacuum chambers. Depending on coating characteristics, pumping speeds up to 1 l s 1 cm 2 for H2 and 10 l s 1 cm 2 for CO were measured at room temperature, together with CO surface capacities up to 1016 molecules cm 2 [12]. Ultimate pressure limitations were not detected down to 10 13 Torr, except those resulting from the presence of gases not pumped by NEG (i.e. methane and rare gases) and/or due to the measuring gauge outgassing [6]. This technology was developed for the demanding vacuum needs of particle accelerators, for which it provides a high, evenly distributed
0042-207X/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.022
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pumping speed, with the additional unexpected bonus of very low secondary electron yields [13]. However, NEG coatings could be conveniently used for other UHV applications, as for instance to improve the performance of turbomolecular and sputter-ion pumps, or for standard UHV components. Furthermore, they could also be adopted for electronic devices and more generally for any vacuum sealed device such as cryogenic dewars/transfer lines or evacuated solar collectors. All the materials tested are based on the elements of the IV-B column of the periodic table, i.e. Ti, Zr and Hf. These elements provide oxygen solubility limits higher than 20% (atomic), i.e. higher by more than an order of magnitude than those of any other element at room temperature. Due to high binding energy, oxygen is not released by getters while heating. Therefore, repeated activation/atmospheric exposure cycles result in a progressive oxygen enrichment of the coating, which in turn limits its operating life. When assuming that the surface passivation layer consequent to atmospheric exposure contains the equivalent of 10 monolayers of oxygen, which is dissolved inside the coating during activation, an oxygen content at the percent level is established after five activation cycles of a film 1 mm thick. Although also grain boundaries may participate to the oxygen uptake, the importance of a high solubility limit for oxygen is evident. Due to the practical importance of these considerations, the performance variation (ageing) of NEG films, resulting from repeated activation/ air venting cycles was extensively investigated. It was shown that a 5 mm thick TiZrV film may undergo more than 50 such cycles with minor consequences [10], provided that the activation temperature is progressively increased from 200 C to 350 C. Although reassuring, this result does not remove the problem. To do so, materials with little or no reactivity for O2, N2, H2O would be required. In this case not only the coating life would be unlimited, but also heating would not be mandatory for activation. The obvious price to be paid, however, would be a highly selective pumping. Materials with these properties actually exist inside the family of noble metals. In particular,
palladium presents a negligible enthalpy of reaction with CO2 and N2, while its low value for CO and H2 allows these gases to be desorbed by heating. Palladium is particularly interesting also for its extremely high diffusivity and absorption capacity for H2, and therefore it has been privileged in our study. Platinum has also been submitted to exploratory tests, and the available results will be reported for comparison.
2. Experimental techniques Coating was carried out by magnetron sputtering making use of a central cathode consisting of single wires 1 mm in diameter. Typical coating parameters were cathode voltage 500 V, discharge gas (argon) pressure 2 10 2 Torr, applied magnetic field 200 G, substrate temperature 100 C, ( s 1 and film thickness about coating rate 0.1 A 0.5 mm. A more detailed description of the coating process may be found in Ref. [1]. Samples of different size and geometry (small samples for surface analysis and cylindrical tubes for vacuum performance evaluation) were analyzed by Auger spectroscopy, electron microscopy, pumping speed measurements and electron stimulated desorption (ESD) of neutrals. As for the production procedure, the evaluation techniques were the same already used for NEG coatings and described in more details in Ref. [1].
3. Results 3.1. Surface analysis As expected for noble metals, the Auger spectra of the air-exposed Pd films are widely different from those of most metals. The O-KLL line at around 515 eV, produced by the surface oxide, is in this case absent, even for as-received, not heated samples, showing that a stable surface passivation layer is not formed on Pd upon atmospheric exposure. However, a carbon surface contamination may be detected, very similar to that usually noticed for metal surfaces of different nature.
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3.2. ESD Important differences are also visible in the ESD results, shown in Fig. 1. In this figure the total pressure increase and the effective desorption yield variations as a function of the heating cycles for Pd, TiZr and stainless steel are compared. In all cases, the temperature dependence is much less pronounced for Pd than for the TiZr film, without any indication of activation onset. The absolute values for H2 are extremely low, while for CH4 they are similar to those of activated TiZr. The CO2 yields are much higher, also due to the lack of pumping for this gas, while the yields of CO are not measurable because in the shadow of the CO2 cracking pattern. Very
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similar results (within a factor of 2) were obtained for Pt. The origin of the dominant desorption yield for CO2 deserves a complementary comment. Since this gas is not chemisorbed by Pd, its release cannot be imputed to a direct desorption mechanism, but rather to an electron-induced cracking of the carbonaceous surface contaminants. If the sample is not heated (see Fig. 2) the desorption yields are initially about one order of magnitude higher than those measured after 120 C heating, and they decrease with increasing pumping time, reaching values 2–5 times higher after 50 h of pumping. Also in this case very similar results were obtained from Pt coatings.
Fig. 1. Dependence of Pd coating ESD results on heating temperature (heating duration 2 h), compared to that from stainless steel and TiZr coating. The samples are bare and coated stainless steel cylinders 50 cm long and 10 cm in diameter. The energy of the bombarding electrons is 500 eV, the electron current 1 mA. The reported data are apparatus-dependent whenever sample surface pumping takes place because the competing action of desorption and sample pumping are not disentangled.
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160 mm diameter, connected at the other extremity to an UHV vacuum system via an orifice of 25 l s 1 of conductance for H2. A detailed description of this system is given in Ref. [6]. The pressure measured after 24 h baking at 250 C was 2 10 13 Torr. The corresponding sticking factor for H2 was higher than 0.15. This value was obtained by Monte Carlo simulation from the pressure attenuation across the chamber measured when the sample gas is injected at one of its extremities. Since only total pressure is measured, the presence of impurities cannot be excluded. If the impurities are not pumped by the Pd film, they adversely affect the pressure gradient and decrease the calculated sticking probability. This effect is more important for long, narrow chambers and injected gases pumped with high speed. Since both these condition are present in our case, only a lower limit of the sticking probability could be given. For comparison, H2 sticking probabilities of 0.5 are reported [15]. For the same reason, the sticking probability for CO could not be obtained; according to literature, it is very close to 1 [14]. Pumping speeds for H2 were also measured for the unbaked Pd-coated chamber after different pumping times. Fig. 3 shows the H2 pumping speed versus water vapour pressure, which steadily decreases while pumping. This result indicates that heating of Pd thin films is not required for H2 pumping.
4. Pd coatings on NEG films
Fig. 2. Results obtained as described for Fig. 1 after different pumping times without sample heating. The horizontal lines correspond to the values measured after heating at 120 C.
3.3. Ultimate pressure and pumping speeds The ultimate pressure was measured at the extremity of a 2 m long Pd-coated chamber of
Although large H2 quantities may be accommodated in Pd at high pressures, due to its low binding energies both for surface adsorption (about 1 eV) [15] and for solid solution (about 0.1 eV) [16], even small H2 quantities result in dissociation pressures unacceptably high if compared to UHV standards. A better situation might be achieved if the Pd film is coated over a predeposited NEG film as already reported for Pd coatings on bulk Nb and Ta [17]. In this case H2 could migrate through the Pd to the NEG film, where it may be stored with a negligibly low dissociation pressure. For this
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Fig. 3. Hydrogen pumping speed dependence on the water vapour pressure for the Pd coated sample of Fig. 1. The measurements were carried out using a Fisher–Mommsen pumping speed dome after different pumping times in an unbaked vacuum system. Care has been taken to minimize the injected amount of H2 for pumping speed evaluation, so as not to affect appreciably the H2 surface coverage which in turn would result in a pumping speed reduction for the following measurements.
application, Pd might be advantageously replaced by PdAg alloys, which provide an easier transmission of H2 to the underlying NEG [18]. In order to explore this possibility, stainless steel vacuum chambers coated with Pd, Pd/NEG, and Pd70-Ag30/NEG, were exposed to various H2 loads (after bakeout at 120 C for 24 h) and then heated under static vacuum with a ramping rate of 60 C/h. The results are shown in Fig. 4. The Pd film coated on stainless steel, even without any H2 injection, displays a steady pressure increase for increasing temperature. At distinction with this behaviour, in the case of Pd coated on a TiZr film, heating produces initially an increase of the H2 pressure, which then decreases down to the ultimate pressure of the system. This trend is common to the H2 loaded and unloaded Pd/NEG coated chambers, although with a slightly different H2 pressure peak temperature, showing that H2 may be fully transferred at temperatures lower than 100 C. On the other hand, no pressure increase is detected for PdAg/NEG, in spite of the much larger injected H2 quantity, showing that no heating is needed in this case for transferring
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Fig. 4. Temperature dependence of the H2 pressure measured for different samples (same geometry as those of Fig. 1) submitted to different H2 loads and heated under static vacuum at a rate of 60 C/h.
H2 to the NEG film due to the low activation barrier for H2 migration from the surface into the PdAg bulk and from the latter into the TiZr film.
5. Conclusions On palladium thin film coatings exposed to ambient atmosphere a stable passivation layer is not formed. The low binding energies for H2 and CO make their pumping thermally reversible, so allowing a practically unlimited operating life to be achieved. A similar behaviour is also displayed by Pt films, which however have not been studied at the same extent. In order to compensate for the low H2 pumping capacity in the UHV pressure range Pd and PdAg coatings have been used as overlayers on NEG films. In this case both the higher sticking factor for H2 of Pd and the large H2 absorption capacity of NEGs are jointly available in a hybrid coating, which is ageing free and does not require heating for activation. Such features make Pd/NEG and PdAg/NEG particularly suited for clean UHV systems which
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are often vented to air and/or for pumping large quantities of H2 in unbaked vacuum systems, where water vapour would otherwise completely saturate the NEG film. In this case, however, the coating should be kept warm enough during operation to prevent water vapour physisorption (about 100 C). For particle accelerator applications, the hybrid coatings provide lower electron-induced desorption yields than the usual construction materials, and even than NEG coatings in the case of H2. On the other hand, no pumping is provided for CO2, and the secondary electron yields are intrinsically high (peak value of 1.4 even after heating at 350 C [19]). Before an application to a real machine could be envisaged, more work would be needed, particularly to explore the dynamic vacuum behaviour under electron/synchrotron radiation surface bombardment and the possible benefits of specific treatments to decrease CO2 desorption.
References [1] Benvenuti C, Chiggiato P, Cicoira F, L’Aminot Y. J Vac Sci Technol A 1998;16:148. [2] Benvenuti C, Chiggiato P, Cicoira F, Ruzinov V. Vacuum 1998;50:57. [3] Benvenuti C, Proceedings of EPAC 1998. URL: http:// accelconf.web.cern.ch/AccelConf/e98/PAPERS/ THZ02A.PDF.
[4] Benvenuti C, Cazeneuve JM, Chiggiato P, Cicoira F, Escudeiro Santana A, Johanek V, Ruzinov V, Fraxedas J. Vacuum 1999;53:219. [5] Benvenuti C, Chiggiato P, Costa Pinto P, Escudeiro Santana A, Hedley T, Mongelluzzo A, Ruzinov V, Wevers I. Vacuum 2001;60:57. [6] Benvenuti C, Escudeiro Santana A, Ruzinov V. Vacuum 2001;60:279. [7] Prodromides A, Scheuerline C, Taborelli M. Vacuum 2001;60:35. [8] Chiggiato P, Kersevan R. Vacuum 2001;60:67. [9] Benvenuti C, Chiggiato P, Mongelluzzo A, Prodromides A, Ruzinov V, Scheuerlein C, Taborelli M, L!evy F. J Vac Sci Technol A 2001;19:2925. [10] Benvenuti C, Proceedings of PAC 2001. URL: http:// accelconf.web.cern.ch/AccelConf/p01/PAPERS/ ROPB002.PDF. [11] Collins IR, Ruzinov V, Malyshev OB, Anashin V, Dostovalov R, Fedorov N, Krashov A, Proceedings of EPAC 2002. URL: http://accelconf.web.cern.ch/AccelConf/e02/PAPERS/WEPDO013.pdf. [12] Benvenuti C, Chiggiato P, Costa Pinto P, Prodromides A, Ruzinov V. Vacuum 2003;71:307. [13] Henrist B, Hilleret N, Scheuerlein C, Taborelli M. Appl Surf Sci 2001;172:95. . [14] Conrad H, Ertl G, Koppers J. Surf Sci 1978;76:323. [15] Behm RJ, Christmann K, Ertl G. Surf Sci 1980;99:320. . G. Physics [16] Jehn H, Speck H, Hehn W, Fromm E, Horz data, gases and carbon in metals, Pt XIX. Karlsruhe: Fachinformationszentrum Energie Physics Mathematik GmbH, 1981. p. 7. [17] Pick MA, Davenport JW, Strongin M, Dienes GJ. Phys Rev Lett 1979;43:286. [18] Noordermeer A, Kok GA, Nieuwenhuys BE. Surf Sci 1986;165:375. [19] Hilleret N. (CERN). Private communication.